1. Field of the Invention
The present invention relates generally to electronic fingerprint sensors, and more particularly to a fingerprint sensor using a thin-film transistor (“TFT”) array to capacitively sense a user's fingerprint.
2. Description of the Related Art
Conventional fingerprint sensors currently being sourced to the commercial market use different methods for sensing a user's fingerprint. One type of fingerprint sensor includes a CMOS silicon chip having circuitry providing an arrayed multitude of “pixels”. The CMOS silicon chip is then coated with a protective coating that may be formed from a simple chemical coating, a flex substrate, or other thin materials. This type of fingerprint sensor requires the silicon chip to be at least as large as the pixel array.
A second type of commercially available fingerprint sensor includes metal lines formed upon a substrate to form a pixel array, while a remotely-located silicon chip, of smaller dimensions than the pixel array, is electrically coupled thereto. This second type of fingerprint sensor can be implemented in several different packaging configurations, such as ball grid array (BGA), wafer level fan-out (WLFO), or a film substrate conformed around, or on top of, a plastic hump/stiffener.
Fingerprints are characterized by patterns of ridges and valleys that are present on the skin of a user's finger. Most of the current commercial fingerprint sensors are capacitive touch sensors, meaning that the circuitry used to derive a fingerprint image must be capable of differentiating small changes in a received signal that result from the capacitance induced by a finger “ridge” or “valley” positioned over the plate of a capacitive sensing element. These capacitive sensing elements are typically laid out in an array of X rows by Y columns, commonly referred to as a “pixel array”. The intersection of each row with each column is referred to as a “pixel”. These pixel arrays can be created by CMOS devices formed in a semiconductor integrated circuit chip itself, as in the first type of fingerprint sensor described above. Alternatively, the pixel array can be formed by metal lines formed upon on a non-semiconductor substrate material, as in the second type of fingerprint sensor described above.
The first type of fingerprint sensor described above results in a much higher-cost product, since the CMOS silicon chip in which the pixel array is formed must be at least the size of the fingerprint image required. In the case of a touch sensor, or 2D sensor, this can require a relatively large area of silicon, measuring three-quarters of an inch square or larger, making it relatively costly.
On the other hand, the second type of fingerprint sensor described above, wherein metal traces are formed upon a non-semiconductor substrate, often develop an inferior signal due to the limitations on the widths of the lines used to transmit and receive the signal from which the fingerprint image is derived. The smaller sizes of the transmitter and receivers, especially the transmitter, can also severely limit the thickness of material above the sensor.
A third type of fingerprint sensor that has been proposed uses a liquid crystal display (LCD), which is ordinarily used to display, rather than sense, information. In this third type of sensor, the LCD display itself is used to image and capture the fingerprint, providing a single device that is both a display and fingerprint sensor. This method proposes the use of the “column drivers” of the display to not only output information but to have an input mode that can sense a capacitance change on the pixels in the display. This method is extremely limited in signal strength because the column lines must be used as both transmission (Tx) lines and receiver (Rx) lines. For example, it has been proposed to use a “pre-charge” state on each pixel before the user's finger is applied, and then to detect the voltage change on each such pixel in the presence of the user's finger, thereby monitoring the capacitance provided by the ridge or valley of the finger over that pixel. The use of the column line as both Tx (precharge) and Rx (receive, or read) severely limits the signal to noise capabilities that this method can produce. In addition, this method is costly, as all of the column drivers must be designed to serve as both an output device (for normal display usage) and a highly sensitive input device (for fingerprint sensor usage).
In order for the above-described sensors to properly distinguish the “ridge” versus “valley” signal delta, the finger must be located as close as possible to the receiver plate(s) of the capacitor. Accordingly, suppliers of known fingerprint sensors strive to minimize the thickness of the receiver plate that overlies the capacitive plate of each pixel. However, as the receiver plate thickness is reduced, such fingerprint sensors are more easily damaged physically or mechanically because of the close proximity of the sensor surface to underlying electrical circuitry, thus reducing the durability and/or reliability of the sensor. For example, conventional BGA-style fingerprint sensors, as well as newer, more advanced “flexible” fingerprint sensors, which enable a user to swipe a finger across a polyimide surface without directly contacting the sensor circuitry, are both susceptible to this type of damage.
As explained above, current fingerprint sensors require that the user's fingertip be in close proximity to the fingerprint sensor circuitry in order to sufficiently distinguish the ridges and valleys of the fingertip. Accordingly, for the second type of fingerprint sensor described above, the thickness and material type used to protect the fingerprint sensor is severely limited. The protective coatings currently used to cover fingerprint sensors must be non-conductive, less than approximately 200 um thickness, and fit the aesthetic requirements of the customer. For example, a simple drop of a pen striking the exposed portion of the fingerprint sensor can damage the thin polyimide surface of the flexible fingerprint sensor, thus creating aesthetic defects and potentially damaging the sensor circuitry located just below the surface. The ability to place thicker materials over the sensor to add to the reliability of the fingerprint sensor is highly desirable. However, thicker protective coatings/surfaces introduce at least two new challenges: 1)—the signal strength from the signal transmitter, to the finger, then back to the receiver array, is greatly diminished as thickness of the cover increases, typically by the square of the increased distance (i.e., doubling the cover thickness reduces signal strength by a factor of four); and 2)—depending on how the transmitter signal is generated, the transmitter signal can be significantly de-focused as it travels from the transmitter to the receiver.
Among the fingerprint sensors that are currently available is a “Glass Cap Sensor” supplied by Silicon Display of South Korea under Model No. GCS-2. This device provides a poly silicon thin film transistor (TFT) capacitive pixel array of 256 rows by 360 columns, corresponding to 92,160 sensor cells. The pixel density corresponds to 508 dpi, and is provided within a sensing area measuring 12.8 mm by 18 mm. The pitch between successive pixels in the array is 350 micrometers. A gate/row shift register is formed on the integrated circuit and is used to select the active row of pixels to be sensed. Likewise, a column shift register is formed on the integrated circuit for selecting the columns to be sensed within the selected row. Four analog output sensing signals are provided at any given point in time. A multiplexer is also formed on the integrated circuit, and is used to select which column output sensing signals are selected at any given point in time. Applicants believe that the above-described Glass Cap Sensor is essentially a passive device that does not include any signal generating electrodes for radiating a high frequency signal proximate to the pixel array in order to detect the effective capacitance formed between each of the pixels of the array and the user's fingertip.
U.S. Pat. No. 6,055,324 to Fujieda discloses a fingerprint imaging device including a two-dimensional array of thin film transistors (TFTs) formed within a substrate, a dielectric layer formed above such substrate, and signal sensing electrodes formed on the dielectric layer. The signal sensing electrodes are connected to the source terminals of the thin film transistors. The gate electrodes of TFTs lying within the same row of the array are connected to a common gate electrode lead. The gate electrode leads are connected to output terminals of a shift register used to select which of the rows of the array is active. The drain electrodes of TFTs lying in the same column are connected to a common drain electrode lead. The drain electrode leads are connected to input terminals of a signal detecting circuit. A signal generating electrode is provided in the form of mesh or comb for surrounding the pixels of the two-dimensional array and for radiating a high frequency signal toward a finger overlying the array. The signal sensing electrodes of the array form electrostatic capacitances between the signal sensing electrodes and the user's finger. The signal received by each of the signal sensing electrodes is detected, row by row, to provide an image of a fingerprint. However, in Fujieda, the signal generating electrode is so highly enmeshed with each of signal sensing electrodes of the array that significant components of the radiated high frequency signal are directly capacitively coupled to the signal sensing electrodes without first passing through the user's finger. As a result, the difference in signal strength between a first signal sensing electrode lying below a ridge of the user's fingertip, and a second signal sensing electrode lying below a valley of the user's fingertip, is not nearly as pronounced as it should be. Moreover, as the thickness of the protective layer, separating the user's finger from the underlying signal sensing electrodes, is increased, the direct capacitive coupling of the radiated high frequency signal from the signal generating electrode to the array of signal sensing electrodes largely overwhelms any secondary coupling of the radiated high frequency signal through the user's finger.
As evidenced by the purchase of Authentec by Apple, the fingerprint sensor is a biometric security system with great potential in the cell phone, notebook, and laptop arena. Thus, the ability to embed a fingerprint sensor in an LCD panel, or to create a fingerprint sensor in a component which is common to many of these media, such as a button, is highly desirable.
Accordingly, it is an object of the present invention to provide a fingerprint sensor for imaging a person's fingerprint without requiring the use of an integrated circuit semiconductor chip of the same dimensions as the pixel array used to capture the image of the fingerprint.
Another object of the present invention is to provide such a fingerprint sensor which more readily distinguishes between the ridges and valleys of a fingertip applied to a cover plate overlying the pixel array used to image the fingerprint.
Still another object of the present invention is to provide such a fingerprint sensor wherein the cover layer, or coating, overlying the pixel array can be made of sufficient thickness to adequately protect the pixel array while still permitting the pixel array to readily distinguish between the ridges and valleys of an applied fingertip.
A further object of the present invention is to provide such a fingerprint sensor which can be manufactured at relatively low cost.
Yet a further object of the present invention is to provide such a fingerprint sensor which more effectively transmits a carrier electrical signal into the person's fingertip without simultaneously directly coupling such carrier signal into the pixel array.
A still further object of the present invention is to provide such a fingerprint sensor which reduces the number of electrical lines between the pixel array and an associated integrated circuit used to process the fingerprint image captured by the pixel array.
Yet another object of the present invention is to provide such a fingerprint sensor wherein the signal components monitored by each pixel within the pixel array can be sensed differentially to reject common mode noise signals.
Still another object of the present invention is to provide such a fingerprint sensor wherein the pixel array may be incorporated as a portion of a conventional touch-sensitive pad.
Another object of the present invention is to provide a fingerprint sensor which readily transmits a signal into the user's finger that can be sensed by the pixel array, but wherein the transmitted signal is not significantly directly coupled to the pixel array through the fingerprint sensor itself.
A still further object of the present invention is to provide a fingerprint sensor which can be easily combined with a conventional touchpad to provide a single device which can both image a user's fingerprint and detect that the user is touching a particular location of the touchpad, within the same sensing layers.
These and other objects of the present invention will become more apparent to those skilled in the art as the description of the present invention proceeds.